Path to strong insulating transport properties in bulk AlPdRe samples

Electronic transport measurements have been made on bulk icosahedral Al₇₀Pd_22.5Re_7.5 quasicrystal (QC's) samples, having increasingly larger resistance temperature ratios, r_T = R(4.2 K)/R(292 K). Data were taken between 0.023 K to 292 K and in magnetic fields up to 17.9 T. Both the zero field resistivity and the magnetoresistance (MR) changed from metallic behavior to weakly insulating behavior to highly insulating behavior, as the resistance temperature ratios r_T's of the samples were made larger. For the insulating samples, the resistivities ρ's followed simple inverse temperature power laws above 50 K going as ρ(T) = a₀/T^z, where z = 1 ± 0.1. The insulating QC samples exhibited saturation behaviors of their resistivities below 2 K. Below 0.3 K, the strongly insulating QC's displayed activated variable-range hopping (VRH) laws in their conductivity; the hopping exponents y's in the VRH laws varied between 0.19 ≤ y ≤ 0.43. A simple model including conductivity contributions both from the primary insulating QC phase and from a secondary metallic phase yielded good fits to the resistivity and MR data.